Add support for third party policy gems

[NullVoxPopuli]

https://github.com/elabs/pundit
https://github.com/nathanl/authority
etc

Pundit looks super close to what I have, so maybe I could break out skinny_controller's policies to their own gem, and have some hooks for the other policy gems?

[NullVoxPopuli]

This should be configurable via an initializer config option (just like the existing configuration options)

For something like authority, where the methods have a different naming scheme, this should be picked up automatically with an option to be configured.